Many manufacturers, but particularly those who manufacture high density integrated circuits are interested in constructing very small patterns in a thin film known as a resist. These films are typically 1 to 2 .mu.m thick on the substrate to be patterned, and the size of the desired pattern may be as small as 1 .mu.m or less. One method of forming these patterns involves the use of photolithography, where the optical image of a mask, having the desired pattern on it, is focused onto the photoresist coated surface of the substrate. When exposed to a developing solution, the photoresist used will either remain behind where it was exposed to light or it will be removed where it was exposed to light. In the former case this is referred to as a negative acting resist, while in the latter case it is known as a positive acting resist.
It has been observed that existing negative acting photoresist films have a distinct tendency to swell during the resist's development because organic solvents are employed as developers. As a result, it is difficult to form patterns having a narrow open gap between two pads of resist because the resist remaining behind swells such that the narrow gap is bridged over, i.e., the resist from the two, supposedly separate, pads will touch each other during development. Even though the resist film will shrink after being removed from the developer, in the locations where swollen resist contacted another part of the pattern a stringy or filmy bridge will be created between the two parts of the pattern.
Because of these swelling problems and because positive acting resists are known not to swell upon development, it has become accepted practice to utilize positive acting resists for high resolution microphotolithography. However, for some patterns it would be more convenient to be able to use a negative tone resist. For example, a mask of a certain pattern may be more easily prepared, or might be more reliable to use if it were mostly dark with a few clear areas rather than if it were the opposite tone, namely, mostly clear with a few dark areas. That this might be the case can readily be imagined by considering the effect of dust particles on a mostly clear mask as opposed to a mostly dark mask. Any dust particle in the mask's clear area will be reproduced in the resist as a defect regardless of the tone of the resist. Therefore, the mostly clear mask has, proportionately, a greater area susceptible to particle contamination than does the mostly dark mask.
It is known that conventional positive acting photoresists comprised of novolac binder resin and diazoketone or diazoquinone photosensitizer can be caused to yield negative tone images as disclosed, for example, in the U.S. Pat. No. 4,104,070 and in L. F. Thompson, C. G. Willson and M. J. Bowden, ACS Symposium Series 219, 117 (1983).
These images are of high quality since there is no swelling of the resist during the development. The transformation (from positive to negative acting) results from the following chemical processes: ##STR1##
Initially the resist film is insoluble in aqueous developer due to the hydrophobic character of sensitizer (1) (the novolac resin itself is hydrophilic and already soluble in the developer). Upon being converted to photoproduct (2), the sensitizer is transformed into an indene carboxylic acid which is hydrophilic and is itself soluble in the developer solution. Since the novolac resin is already soluble, this causes the resist film to become soluble in the developer where it was exposed to light. However, in the image reversal process, the substrate is heated, in the presence of a catalyst such as Monazoline or ammonia vapor, so that the carboxylic acid (2) is decarboxylated to give an indene compound (3) which is hydrophobic and which makes the film insoluble again. This process was described in Semiconductor International, April 1987, at pages 88-89. However, it is suggested that the use of amines neutralizes carboxylic acids to make the sensitizer (photoacid) less soluble. Actually, the amine salt would be quite soluble in the developer and, therefore, the decarboxylation process referred to above must proceed to the final indene product to render the irradiated areas insoluble. After making the exposed portion of the film insoluble, the entire resist film is flood exposed to light without any image. During this exposure step, the areas previously unexposed during the imagewise exposure are rendered soluble as the sensitizer (1) is converted into carboxylic acid (2). The areas which previously had been exposed to light remain hydrophobic and insoluble since in these regions the sensitizer has already been consumed. All examples of this type of image reversal utilize a base soluble novolac resin whose dissolution is controlled by the diazoquinone sensitizer. If the sensitizer is in a hydrophobic state, the film is insoluble, whereas if it is in a hydrophilic state, the film is soluble. Therefore, in order for the image reversal process to work, the decarboxylation reaction [(2).fwdarw.(3)] must proceed for the sensitizer (and thus the film) to revert to an insoluble state.
It also has been demonstrated that no final flood exposure is needed if the sensitizer is chosen such that its acid photoproduct is capable of cross-linking the exposed areas during a high temperature post-exposure bake. This type of image reversal does not utilize an amine treatment. (Mechanism and Lithographic Evaluation of Image Reversal in AZ.RTM.5214 Photoresist, M. Spak, et al., Proceedings of the SPE Regional Technical Conference on "Photopolymers: Principles, Processes and Materials", Ellenville, N.Y. (1985), p. 247-269.)
Thus, image reversal processes used with hydrophilic novolac resins and diazoquinone sensitizers produce an insoluble hydrophobic region by decarboxylation of indene carboxylic acids or by acid-catalyzed cross-linking. (See R. M. R. Gijsen, et al., "A Quantitative Assessment of Image Reversal, A Candidate for a Submicron Process with Improved Linewidth Control," SPIE Vol. 631, Advances in Resist Technology and Processing III, (1986), p. 108-116).
Conventional novolac photoresists are limited in that they cannot be exposed at short ultraviolet wavelengths, .ltoreq.300 nm, to give high resolution images because of the intense absorbancy of novolac resin in this region. Positive acting resists have been developed to circumvent this problem (e.g., U.S. Pat. No. 4,491,628 and U.S. Application Ser. No. 832,116) by using different sensitizers and different binder resins. The sensitizers are compounds such as onium salts which produce a strong acid upon photolysis, while the resins are polymers selected for their UV-transparency and other physical properties. The resins have as an integral part an acid labile group, the removal of which is capable of transforming the polymer from insoluble in aqueous developer to soluble in aqueous developer. It has been claimed (U.S. Pat. No. 4,491,628) that such resists may be processed to give a negative tone image by using an organic, non-polar solvent as the developer rather than the aqueous alkali used to prepare positive tone images. This has the disadvantage of requiring the use of materials more difficult to handle, due to their flammability, toxicity or volatility as compared to those of the positive tone processing conditions. These organic developers also reintroduce the possibility of swelling and crazing of the polymer films.